<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <link rel="stylesheet" href="">
    <link rel="stylesheet" href="">
    <link rel="stylesheet" href="">
    <title>myArrayFrom</title>
</head>
<body>
<script>
    // arrayLike
    // 1. object
    // 2. 索引
    // 3. length
    // 4, 能被迭代
    // polyfill
    // a. 对于对象length的处理，
    // 比如 NaN => 0,Infinite=>Infinite 0=>0 , 取整(-2.5 => -2 , 2.5=> 2)。
    // 实际长度为 <0 ,取 0。最大安全整数：Math.pow(2, 53) - 1 和 实际长度的最小值。
    // b.对于第二个参数，必须为function
    // c.传入的 arrayLike 不能是 null undefined
    // 当前是不是函数是的话 new C() 否则 new Array()
    // while遍历arrayLike，有 mapFn  mapFn(kValue,k) 如果有this mapFn.call(T,kValue,k)
    // 将arrayLike的length属性设置到 最后结果返回数组的 length
    // 返回数组
    Function.prototype.myArrayFrom = (function () {
        var toStr = Object.prototype.toString;
        var isCallable = function (fn) {
            return typeof fn === 'function' || toStr.call(fn) === '[object Function]';
        };

        var toInteger = function (value) {
            var number = Number(value);
            if (isNaN(number)) { return 0; }
            if (number === 0 || !isFinite(number)) { return number; }
            return (number > 0 ? 1 : -1) * Math.floor(Math.abs(number));
        };
        var maxSafeInteger = Math.pow(2, 53) - 1;
        var toLength = function (value) {
            var len = toInteger(value);
            return Math.min(Math.max(len, 0), maxSafeInteger);
        };

        return function (arrayLike, /*mapFn, thisArg*/) {
            // 1. Let C be the this value
            let C = this;

            // 2. Let items be ToObject(arrayLike).
            let items = Object(arrayLike);

            // 3. ReturnIfAbrupt(items).
            if (arrayLike == null) {
                throw new TypeError("Array.from requires an array-like object - not null or undefined");
            }

            // 4. If mapfn is undefined, then let mapping be false.
            var mapFn = arguments.length > 1 ? arguments[1] : void undefined;

            var T;
            if (typeof mapFn !== "function") {
                // 5. else
                // 5. a If IsCallable(mapfn) is false, throw a TypeError exception.
                if (!isCallable(mapFn)) {
                    throw new TypeError('Array.from: when provided, the second argument must be a function');
                }
                // 5. b. If thisArg was supplied, let T be thisArg; else let T be undefined.
                if (arguments.length > 2) {
                    T = arguments[2];
                }

            }

            // 10. Let lenValue be Get(items, "length").
            // 11. Let len be ToLength(lenValue).
            var len = toLength(items.length);

            // 13. If IsConstructor(C) is true, then
            // 13. a. Let A be the result of calling the [[Construct]] internal method
            // of C with an argument list containing the single item len.
            // 14. a. Else, Let A be ArrayCreate(len).
            var A = isCallable(C) ? Object(new C(len)) : new Array(len);

            // 16. Let k be 0.
            var k = 0;
            // 17. Repeat, while k < len… (also steps a - h)
            var kValue;
            while (k < len) {
                kValue = items[k];
                if (mapFn) {
                    A[k] = typeof T === 'undefined' ? mapFn(kValue, k) : mapFn.call(T, kValue, k);
                } else {
                    A[k] = kValue;
                }
                k += 1;
            }
            // 18. Let putStatus be Put(A, "length", len, true).
            A.length = len;
            // 20. Return A.
            return A;
            /*var array = [];
            for (var i = 0; i < arrayLike.length; i++) {
                array.push(arrayLike[i])
            }
            return array*/
        }
    })()

    const doms = document.querySelectorAll("link")
    console.log("====================");
    console.log("doms", doms)
    console.log("array", Array.myArrayFrom(doms))

</script>
</body>
</html>
